The most basic part of a modular standard is the frame that supports the track work and scenery.
There are two basic designs that predominate in the design of frames for modules.
The first one is a simple box with a top and no bottom. The sides of the box are wood and the top used to be plywood but now is most likely foamboard. This is what Linn Westcott in his book How to Build Model Railroad Benchwork refers to as butt-joint benchwork.
The other design is two end plates attatched to a top plate and conformable side plates usually of masonite. It is also a simple box but has more free-flowing design capabilities.
What they share in common is that the frame is supposed to be fairly rigid to keep the track work at the proper angle to the floor and to other track work. The old frames with plywood tops were fairly rigid because you had a five sided wooden box with a lot of nails and/ or screws holding it together. The new style box is only four sided since the top is usually just plopped on. I see this all the time. The builder may put braces around the outside edge to keep the foam in place (and they think from sagging–but they are wrong) but rarely do you see any real cross-bracing or even “bed slats” to keep the foam from sagging. They flex and flop like a dead herring.
In fact, without a lot of crossbracing which is heavy and expensive, all of the boxes flex. They are usually pretty rigid end to end and fairly rigid on the diagonal. Where they have problems is twisting. Take a module and try to twist it so that the two interace plates are no longer parallel to the floor. It is usually not hard. The free-flowing designs are usually more rigid than their slab sided brethren because the tops are usually thicker, the sides wider, and more cross-bracing used.
Is this flexing a problem?
Yes, from a track reliability standpoint but in fact many modules rely on this feature (even if they don’t realize it) to align modules to each other. I would say that the majority of modules in a meet are actually in torsion during the entire meet. Let’s see how this happens with a typical four legged butt-joint module. You put your module next to another module and adjust the leg levelers until the two modules match, then clamp them together rigidly. One or the other leg on the other end of the module may not be touching the floor when you are done. It probably is though because the weight of the module is enough to flex it. This not only reduces the reliability of the track but it weakens the frame over time.
How do you prevent this, aside from an entirely different approach to frame design? First, build your module narrow. The more narrow the module is the less force can be applied to flex the module. Second, don’t make it slab sided. Curves are harder to flex than straight pieces. Third, rely more on the top plate and the cross-braces and less on the sides (I will have much more to say about legs in a separate article–for now assume that legs are NOT part of the frame). Butt-joints are inherently weak, just adding a filler in each corner greatly improves a simple butt-joint frame. Using the leg as this filler is not a good idea. Legs are subjected to all sorts of flexon. If you rely on them to act as corner braces they will begin to work loose and you will no longer have an effective corner brace.
More on this later. Especially on alternate designs and materials.